1. Field of the Invention
The present invention relates to indexable inserts used for turning. In particular, the present invention relates to an indexable insert that provides enhanced durability of a breaker projection while it maintains favorable chip processability so as to minimize deterioration of the chip processability at an early stage.
2. Description of the Related Art
A known indexable insert (sometimes simply referred to as an “insert” hereinafter) for turning, which is used by being attached to a tool holder, is provided with chip breakers on a rake face of the insert to enhance chip processability. The function of a chip breaker is to cut a chip formed during turning to an appropriate length so as to separate the chip from the workpiece and the cutting tool. Chip breakers having various shapes have been proposed, including those disclosed in Japanese Patent No. 3607288 and Japanese Unexamined Patent Application Publication No. 2002-210604.
The chip breakers disclosed in Japanese Patent No. 3607288 and Japanese Unexamined Patent Application Publication No. 2002-210604 are of the most commonly used projected type. In Japanese Patent No. 3607288, breaker projections are provided on the insert face (i.e., the rake face) at positions adjoining the corners of the insert. Each breaker projection has a curved shape having a concave side facing towards the corresponding corner of the insert and a convex side facing away from the corner.
On the other hand, in Japanese Unexamined Patent Application Publication No. 2002-210604, a first breaker projection and second breaker projections are provided on the rake face. The first breaker projection extends from a boss surface (i.e., a central land portion) toward the respective noses. The second breaker projections each extend from an end of the first breaker projection to a slope continuing from a cutting-edge land of the corresponding nose. In plan view, the second breaker projections at each nose have a concave shape along linear cutting edges and are disposed symmetrically with respect to a bisecting line of the nose.
As mentioned on lines 24 to 29 on column 4 of Japanese Unexamined Patent Application Publication No. 2002-210604, at each of the noses, a rising surface extending from a breaker-groove base of a breaker groove is preferably formed from a midsection of the slope, and the second breaker projections are each formed from the midsection of the slope, as mentioned on lines 49 to 50 on column 4 of the publication.
FIG. 11 is a cross-sectional view of a projected-type breaker of the related art, taken along a bisecting line of an apex angle of a nose. A solid line shown in FIG. 11 denotes a chip 20 being removed while sliding on the surface of the breaker. As is apparent from FIG. 11, sections A in the breaker of the related art have a convex shape towards the chip 20. Therefore, the insert and the chip 20 are locally in contact with each other at the sections A, and thus have a small contact area. In the case of a high cutting speed or a large feed, the load applied to the contact regions between the chip 20 and the insert is large, leading to crater abrasion in the contact regions due to a temperature increase or to serious damages caused as a result of detachment of a coating from the insert. These problems can be effectively solved by increasing the length between the cutting edge and the breaker projection to reduce the load received when raking the chip. However, increasing the length between the cutting edge and the breaker projection may deteriorate the chip processability.
In the structure disclosed in Japanese Patent No. 3607288, the breaker projections provided near the noses are used to process chips during large-depth cutting so as to effectively prevent the chips from clogging. As shown in FIG. 12, in common machining, such as external turning, facing, or copy turning, a chip 20 is removed while being curled sideways in plan view. Therefore, with the breaker projections set forth in Japanese Patent No. 3607288, the pressing force applied against the chip 20 is large at section B in FIG. 12, causing the section B to become abraded faster than the other sections. This may result in deterioration of the chip processability at an early stage.
In the structure disclosed in Japanese Unexamined Patent Application Publication No. 2002-210604, a chip formed during small-depth cutting is curled by being restrained by two projections (i.e., left and right second breaker projections) disposed symmetrically with respect to the bisecting line of each nose. Referring to FIG. 13, with regard to a chip formed during large-depth cutting, a chip 20 is processed by being guided from second breaker projections 32 to a first breaker projection 31. However, like the structure disclosed in Japanese Patent No. 3607288, this structure also lacks attention to the protection of the insert. Specifically, as shown in FIG. 13, in the case where the first breaker projection 31 and a rising surface 33 of a breaker groove extend from a midsection of a slope 34, the bottom of the breaker groove forms a V-shape in cross section (FIG. 13) taken along the bisecting line of the corresponding nose. In this case, the chip 20 comes into point contact with the surface of the insert at sections C in FIG. 13. Therefore, the regions of the insert that are in contact with the chip 20 are abraded intensively, leading to chipping of the cutting edge and to reduced chip process ability.